Sort of useful on a computer; way useful on a mobile device. It will sync the articles you choose and save them on the iPhone for offline reading. Again, great for Metro rides. I typically use this in conjunction with Google Reader: when I come across a longer article or blog post that I want to read later, I save it to Instapaper and read it on the train.

There are two versions of Instapaper available in the app store: the free basic version, and the $10 pro version. The pro version includes several extra features, but the most important is that it will remember your place for you. The basic version just drops you at the list of articles every time you start it, which is a pain if you close the app halfway through a long article.

Viewing the notes does require a net connection - it seems that they are cached temporarily but not stored long-term. However, they recently added a feature to mark your favorites notes, which will be saved on your iPhone. I used this for notes about prescriptions and medical information, and information about upcoming flights - basically, stuff that I need to be able to get at whether I have cell reception or not.